The metabolism capability of mammalian cells used in mutation and transformation assays is being increased by infecting the cells with retroviral vectors containing cDNAs coding for drug metabolizing enzymes. The cDNAs for P450s IIA3, IIB1, and IVB1, and a flavin monooxygenase have been inserted into the vectors and infected into C3H 1OT and one-half cells. By Western analysis, all p450s and the flavin monooxygenase are expressed in the 1OT and one-half cells. Furthermore, cells containing IIB1 have increased sensitivity to the cytoxic effects of dimethylnitrosamine and acetylaminofluorene, and cells containing IVB1 show increased cytotoxic effects to aminofluorene. Cytochrome P450 IIA3 is detected by coumarin hydroxylase activity and should activate the carcinogens, dimethyl- and diethylnitrosamine. The studies are continuing to improve enzyme expression and to make the cells responsive to the genetic toxicity of a wider variety of environmental chemicals and to mimic carcinogen activation as it occurs in vivo.